Foamed gel for permeability reduction or mobility control in a subterranean hydrocarbon-bearing formation

ABSTRACT

A process employing a foamed gel in a subterranean hydrocarbon-bearing formation to reduce the permeability of a treatment region therein or control the mobility of a drive fluid in the formation. The foamed gel is formed from a crosslinkable carboxylate-containing polymer, a crosslinking agent containing a reactive transition metal cation, a polyvinyl alcohol, an aqueous solvent, and an added gas.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a process for permeability reduction ormobility control in a subterranean hydrocarbon-bearing formation andmore particularly to such a process, wherein a foamed gel is employed asa permeability-reducing or mobility-controlling composition.

2. Description of Related Art

Foamed gels are known to have a number of oilfield applications. Forexample, U.S. Pat. No. 5,322,125 to Sydansk teaches a process employinga foamed gel as a permeability-reducing agent to reduce gas coning in amatrix environment of a subterranean hydrocarbon-bearing formation. Thefoamed gel of U.S. Pat. No. 5,322,125 comprises a crosslinked polymergel, a foaming gas and a surfactant to induce foam formation. U.S. Pat.Nos. 4,300,634 and 4,389,320 both to Clampitt also teach the use of afoamed gel as a permeability-reducing agent in a subterraneanhydrocarbon-bearing formation. Despite the prior art knowledge of foamedgels having utility in oilfield applications, a need remains foroilfield applications employing alternate foamed gels other than thoseknown in the art that have satisfactory performance properties.

Accordingly, it is an object of the present invention to provide analternate foamed gel that performs effectively in permeability reductiontreatments. More particularly, it is an object of the present inventionto provide a permeability-reducing composition that effectively reducesthe permeability of a desired treatment region under a broad range ofsubterranean conditions. It is another object of the present inventionto provide a foamed gel that effectively controls the mobility of adrive fluid during a hydrocarbon recovery operation. More particularly,it is an object of the present invention to provide amobility-controlling composition that effectively reduces the mobilityof a drive fluid under a broad range of subterranean conditions. It isstill another object of the present invention to provide apermeability-reducing or mobility-controlling composition that isrelatively stable under harsh formation conditions including thepresence of high temperatures, crude oil, high salinity brines or highhardness brines. It is further an object of the present invention toprovide such a permeability-reducing or mobility-controlling compositionthat is cost effective, having practical economic utility in the field.

SUMMARY OF THE INVENTION

The present invention is a process and composition for improvinghydrocarbon recovery from a subterranean formation penetrated by a wellbore. In a first embodiment, improved hydrocarbon recovery is achievedby employing the composition of the present invention to reduce thepermeability of a subterranean formation, and in particular to reducethe permeability of a desired treatment region within the formation. Ina second embodiment, improved hydrocarbon recovery is achieved byemploying the composition of the present invention to control themobility of an oil-recovery drive fluid within a subterranean formation,and in particular to reduce the mobility of the drive fluid within theformation. Although the process of the present invention is described inthe context of two alternate embodiments for permeability reductionapplications and mobility control applications, respectively, it isnoted that the processes of the two embodiments can be practicedsimultaneously within a given formation using the same composition tosimultaneously achieve both permeability reduction and mobility control.Alternatively, the process of either embodiment can be practicedseparately as a solo procedure within a given formation to achievepermeability reduction or mobility control, as desired.

The composition of the present invention is a foamed gel comprising acrosslinkable polymer, a crosslinking agent, a polyvinyl alcohol, anaqueous solvent, and an added gas. The crosslinkable polymer is abiopolymer or a synthetic polymer. A preferred crosslinkable polymer isa carboxylate-containing polymer having one or more carboxylatecrosslinking sites. The crosslinking agent includes a reactivetransition metal cation in the presence of one or more organic and/orinorganic anions. In particular, the crosslinking agent is a saltcomprising the transition metal cation and one or more organic and/orinorganic anions, or a chemical complex, comprising the transition metalcation associated with one or more organic and/or inorganic ligands.

The crosslinking agent crosslinks the carboxylate-containing polymer andthe polyvinyl alcohol uniformly disperses the added gas throughout theresulting crosslinked polymer network, thereby forming the desiredfoamed gel without unduly diminishing the structure, viscosity, strengthor stability of the crosslinked polymer network. Consequently, thefoamed gel displays permeability reduction or mobility controlcharacteristics approaching those of corresponding conventional unfoamedgels at a substantially reduced chemical cost.

In accordance with the first embodiment, the composition of the presentinvention is a permeability-reducing composition having utility in oneor more of the following subterranean treatments: conformanceimprovement, water shut-off, gas shut-off, water coning and gas coning.When utilized in a conformance improvement treatment, thepermeability-reducing composition improves vertical and arealconformance within a subterranean formation and correspondingly improvesflow profiles and sweep efficiencies of injected or produced fluids inthe formation. In doing so, the composition enhances contact within theformation between injected fluids and produced hydrocarbon fluids,thereby desirably mobilizing the hydrocarbon fluids and facilitatingtheir displacement from the formation. When utilized in a water shut-offor gas shut-off treatment, the permeability-reducing composition ispreferably placed at or near a production well bore to block the flow ofwater or gas into the well bore. When utilized in a water coning or gasconing treatment, the permeability-reducing composition is likewisepreferably placed at or near a production well bore to facilitate theflow of liquid hydrocarbons into the well bore.

The permeability-reducing composition can be placed and maintained in atreatment region of the formation in a flowing or non-flowing state. Assuch, the composition can be placed in the treatment region as apreventative treatment prior to the injection of fluids into orproduction of fluids from the formation. Alternatively, the compositioncan be placed in the treatment region after the injection of fluidstherein or production of fluids therefrom as a remedial treatment. Inany case, the composition exhibits a high degree of structure, strengthand stability within the treatment region.

In accordance with the second embodiment, the composition of the presentinvention is a mobility-controlling composition that promotes improvedsweep efficiency and incremental oil recovery of a gas or liquid drivefluid in a hydrocarbon-bearing formation. The mobility-controllingcomposition is placed in the formation in a flowing state in associationwith the drive fluid. The mobility-controlling composition exhibits ahigh degree of viscosity and stability within the formation in thepresence of the drive fluid. The invention will be further understood,both as to its use and composition, from the accompanying description.

DESCRIPTION OF PREFERRED EMBODIMENTS

A number of specific terms are used throughout the specification todescribe the process of the present invention and are defined asfollows. A "formation" is a subterranean geological structure consistingessentially of "matrix," and in some instances, "anomalies." A"reservoir" is a hydrocarbon-bearing formation. An "anomaly" is a volumewithin a formation having a very high permeability relative to thematrix. The term "anomaly" is inclusive of such highly permeable volumesas fractures, fracture networks, joints, cracks, fissures, vugs, voids,solution channels, caverns, washouts, cavities, and the like. The"matrix" is generally characterized as substantially continuous,sedimentary geological material having a very low permeability relativeto an anomaly. In addition, the matrix is often characterized ascompetent.

The term "wellbore" is defined as a bore hole extending from the earthsurface to a reservoir. Thus, a wellbore is a conduit providing fluidcommunication between the surface and the formation penetrated thereby.The term "well" is synonymous with the term "wellbore." A "gel" or "gelmedium" is a continuous three-dimensional chemically crosslinkedpolymeric network integrating a liquid into the interstices of thenetwork. A "foamed gel" is a composition having a stabilized gasdispersion within a gel medium. The foamed gel structure is made up ofgas bubbles separated from one another by lamellae of interfaciallystabilized films of the gel medium. Other terms used herein havedefinitions in accordance with the conventional usage of a skilledartisan, unless otherwise defined hereafter.

The foamed gel utilized in the present invention comprises acrosslinkable polymer, a crosslinking agent, a polyvinyl alcohol, aliquid solvent, and an added gas. Many crosslinkable polymers, includingsynthetic polymers and biopolymers, are known in the art and haveutility herein. A type of crosslinkable polymer having specificapplicability to the present invention are carboxylate-containingpolymers. Preferred among the carboxylate-containing polymers areacrylamide-containing polymers. Of the acrylamide-containing polymers,the more preferred are polyacrylamide (PA), partially hydrolyzedpolyacrylamide (PHPA), as well as copolymers, terpolymers andtetrapolymers of acrylamide. Exemplary copolymers include copolymers ofacrylamide and acrylate, and copolymers, terpolymers or tetrapolymerscontaining AMPS or vinylpyrrolidone. PA, as defined herein, has fromabout 0 mole % to about 1.0 mole % of its amide groups hydrolyzed.Although 0 mole % hydrolyzed PA initially lacks any carboxylate groups,it can undergo autohydrolysis under the conditions of the presentprocess to generate carboxylate groups, thereby satisfying thedefinition of a carboxylate-containing polymer having utility within thescope of the present invention. PHPA has greater than about 1.0 mole %,but less than 100%, of its amide groups hydrolyzed to carboxylategroups. The average molecular weight of an acrylamide polymer havingutility herein is generally in a range between about 10,000 and about50,000,000, preferably between about 150,000 and about 20,000,000, andmost preferably between about 200,000 and about 15,000,000.

The crosslinking agent of the present invention effectuates chemicalcrosslinking between appropriate sites of the same or two differentpolymer molecules, thereby creating the network structure of the gelmedium. Crosslinking agents having utility herein are generallywater-soluble compositions containing a reactive transition metalcation. Such compositions include organic or inorganic salts of thereactive transition metal cation. Preferred salts are inorganic salts ofa trivalent chromium cation such as chromic trichloride, chromictribromide, chromic nitrate, and chromic sulfate. Other suchcompositions include complexes containing the reactive transition metalcation associated with an organic carboxylate anion or an inorganicanion. The term "organic carboxylate anion" as used herein encompassescarboxylate anionic species including mono-carboxylate anions, such asacetate, poly-carboxylate anions, such as malonate, and substitutedderivatives of carboxylate anions, such as glycolate, lactate andcitrate. The carboxylate anions are generally obtained from thecorresponding acids or salts thereof. Preferred among such complexes arethose including a trivalent chromium cation complexed with thecarboxylate anion. An example of such a preferred complex is thetrivalent chromium cation complexed with the acetate anion to formchromium triacetate as taught in U.S. Pat. No. 4,683,949 incorporatedherein by reference. The chromium triacetate complex can be in the formof, or derived from, solid CrAc₃, solid CrAc₃.H₂ O, solid Cr₃ Ac₇ (OH)₂or a solution labeled "Chromic Acetate 50% Solution" that iscommercially available, for example, from McGean-Rohco Chemical Co.,Inc., 50 Public Square, No. 1250, Cleveland, Ohio 44113-2251, U.S.A.Other examples of complexes having utility herein include chromictriproprionate. Of the above-recited crosslinking agents, the mostpreferred are chromic trichloride and chromic triacetate.

Although less preferred, alternate crosslinking agents include salts andcomplexes of transition metal cations other than the trivalent chromiumcation, such as salts or complexes of a trivalent aluminum cation(particularly in the form of aluminum citrate or aluminum trichloride),or of a trivalent iron cation. Yet another alternate crosslinking agentis a hexavalent chromium cation in a redox system.

The polyvinyl alcohol employed herein is preferably cold water soluble,i.e., soluble in water at a temperature less than about 30° C. Thepolyvinyl alcohol has an average molecular weight range between about1000 and 500,000 and preferably between about 50,000 and 250,000. Thepolyvinyl alcohol is generally at least about 60 mole % hydrolyzed andpreferably at least about 75 mole % hydrolyzed.

The aqueous solvent of the gelation solution is substantially anyaqueous liquid capable of forming a gelation solution from the solvent,crosslinkable polymer, crosslinking agent and polyvinyl alcohol. Theterm "solution" as used herein, in addition to true solutions, isintended to broadly encompass dispersions, emulsions, or any otherhomogeneous mixture of the gelation solution components in the aqueoussolvent. The solvent is preferably a fresh water, although a brine, suchas sea water or produced water from a subterranean formation can alsohave utility herein. Produced water can be advantageous because of itslow-cost availability and because it enables the practitioner to returnthe produced water to the formation, thereby avoiding more costly andless environmentally compatible methods of disposing the produced water.

The added gas may be substantially any gas that is substantiallyunreactive with the above-recited polymer, crosslinking agent, polyvinylalcohol, and solvent components and that is capable of being dispersedthroughout the liquid medium. Examples of added gasses having utilityherein are air, nitrogen, methane, carbon dioxide, nitrous oxide,natural gas, steam, produced gas or flue gas. Nitrogen, natural gas,carbon dioxide or produced gas are preferably used in the production ofthe present foamed gels.

The foamed gel is preferably prepared by initially formulating agelation solution at the surface that is a homogeneous fluid admixtureof the solvent, polymer, crosslinking agent and polyvinyl alcohol. Thepolymer, crosslinking agent, and polyvinyl alcohol can each be providedin the form of a discrete aqueous solution. A biocide, such asformaldehyde, can also be added to the gelation solution to prevent thebiological degradation of the polyvinyl alcohol or crosslinkablecarboxylate-containing polymer in the resulting foamed gel when placedin the formation. Surface admixing to produce the gelation solutionbroadly encompasses batch mixing the components in bulk prior toinjection into the wellbore or mixing the components in-line duringinjection into the wellbore. The crosslinkable carboxylate-containingpolymer concentration of the gelation solution is generally at leastabout 500 ppm, preferably at least about 2,000 ppm, and most preferablywithin a range between about 3,000 ppm and about 100,000 ppm. Thetransition metal cation concentration of the gelation solution isgenerally between about 10 ppm and about 20,000 ppm, and preferablybetween about 20 ppm and about 4,000 ppm. The crosslinking agent ispresent in amounts such that the molar ratio of the carboxylate anionsof the crosslinking agent to the transition metal cations of thegelation solution is preferably between about 0.5:1 and 100:1 andpreferably between about 3:1 and 25:1. The polyvinyl alcoholconcentration in the gelation solution is about 20 ppm to about 100,000ppm, preferably about 50 ppm to about 20,000 ppm, and most preferablyabout 1,000 ppm to about 5,000 ppm. Typically the weight ratio ofpolymer to polyvinyl alcohol in the gelation solution is from about0.5:1 to about 5:1.

Preparation of the foamed gel proceeds by appropriately contacting theadded gas with the aqueous gelation solution in a manner that combinesand mixes the added gas and gelation solution to form a foam. The addedgas can be combined and mixed with the gelation solution by conventionalfoaming means such as sparging, high speed mixing, or simultaneouslyflowing the gas and gel composition through one or more orifices, suchas a screen or a plate with one or more orifices, or through a solidpacking, such as a sandpack or a gravel pack. The location and timing ofthe gas addition step can be any one of several alternatives, includingat the surface prior to injection of the foamed gel into the wellbore,at the surface simultaneous with coinjection of the gelation solutioninto the wellbore or in situ. In any case, the polyvinyl alcohol isfound to possess surfactant-like characteristics, thereby promotingfoaming of the aqueous gelation solution.

The volumetric gas content of the foamed gel, termed foam quality, isexpressed as the volume percent of gas in the foamed gel at a giventemperature and pressure. Foamed gels having utility in the presentprocess generally have a foam quality between about 20% and about 99%,preferably between about 50% and about 98%, and most preferably betweenabout 70% and about 97%. The foam quality of the foamed gel is selectedwithin the limits set forth above as a function of the specificapplication of the foamed gel. Foamed gel strength is often an inversefunction of foam quality. Thus, if a high strength foamed gel isdesired, the foamed gel is usually formulated with a relatively low foamquality. Conversely, if a lower strength, less costly, or lower densityfoamed gel is desired, the foamed gel is usually formulated with ahigher foam quality. It is noted that high strength foamed gels havespecific application as permeability-reducing compositions in formationsexhibiting high permeability matrix or relatively large and/or highlyconductive anomalies, such as large fracture networks.

Maturation of the foamed gel entails chemical crosslinking of thepolymer and crosslinking agent within the gelation solution. Foracrylamide polymers containing carboxylate groups, crosslinking, orgelation as it is alternatively termed, is normally initiated as soon asthe polymer and crosslinking agent contact unless an optional delayingagent is included in the gelation solution. The delaying agent can delaythe availability of the crosslinking agent or simply slow the rate ofthe crosslinking reaction. Delaying agents that may have utility in thegelation solution of the present invention are water-soluble acids andsalts of carboxylate anions. The carboxylate anion of an acid or saltdelaying agent can be the same species as the carboxylate anion of thecrosslinking agent or can be a different species. Preferred delayingagents in the practice of the present invention are salts or acids ofacetate, lactate, malonate, citrate, and glycolate. Among the preferreddelaying agents are lactate salts, such as ammonium lactate, potassiumlactate, or sodium lactate. Of the preferred delaying agents, malonateand citrate have the highest degree of ligand strength and acetate thelowest with glycolate and lactate being of intermediate strength,wherein the delay time is normally a direct function of the ligandstrength and concentration of the delaying agent.

For unhydrolyzed PA, crosslinking does not normally occur until the PAhas hydrolyzed to a significant degree. Thus, in the absence of adelaying agent, crosslinking of the unhydrolyzed PA is a function of thepolymer hydrolysis reaction. If a delaying agent is present, however,crosslinking can be a function of both the polymer hydrolysis reactionand the ligand strength of the delaying agent. Where the delaying agentis a carboxylate ligand, crosslinking is also a function of the molarratio of the carboxylate anion to transition metal cation for a givencarboxylate anion. In particular, the length of the delay time for thecrosslinking reaction increases as the molar ratio of carboxylate totransition metal cations increases.

Before crosslinking is completed, the foamed gel is characterized asbeing immature. Once crosslinking has gone to completion, i.e., wheneither substantially all of the crosslinking agent or substantially allof the carboxylate crosslinking sites is consumed, the foamed gel ischaracterized as being mature. The integral components of the resultingmature foamed gel are the gel medium and the added gas dispersedtherein. The crosslinked polymer makes up the structural network of thegel medium and the liquid solvent makes up the interstitial liquid ofthe gel medium.

The physical properties of the foamed gel are a function of the specificfoamed gel components and their relative proportions. Values of theabove-recited variables are selected to create foamed gels across a widerange of viscosities, structures, stabilities, strengths, and densitiesas desired. The viscosity of foamed gels can range from highly viscousto viscosities approaching that of water. The character of the gelmedium contributes to the structure or viscosity and stability of thefoamed gel. In general, the degree of structure or viscosity andstability of a foamed gel containing an acrylamide polymer is increasedby increasing the polymer concentration of the liquid phase. Sometimes,a more cost-effective and preferred means for achieving the same effectis to employ a higher molecular weight polymer or, at other times, apolymer having a higher degree of hydrolysis at a relatively fixedpolymer concentration. Conversely, a reduction in the degree ofstructure or viscosity is achieved by using a lower molecular weightpolymer or, at other times, one having a lower degree of hydrolysis.

It has also been found that the stability and foamability of the foam isat times a function of the molecular weight and concentration of thepolyvinyl alcohol. In general the stability of the foamed gel isenhanced by employing a polyvinyl alcohol having a relatively highmolecular weight, e.g., greater than about 50,000. Similarly, thestability of a foam is enhanced by employing a polyvinyl alcohol at arelatively low concentration in the gelation solution, e.g., less thanabout 12,000. Thus, the skilled practitioner can design the foamed gelin correspondence with the performance requirements of the formation,thereby ensuring the effective performance of the foamed gel as either apermeability-reducing composition or a mobility-controlling composition.

Relative to many conventional oilfield foams, the present foamed gel issignificantly more stable over a wide range of temperature and pressure,as well as formation water salinity, hardness and pH. The foamed gel isalso stable in the presence of liquid hydrocarbons such as crude oil,resisting collapse and fluid drainage.

Use of the foamed gel in a formation as a permeability-reducingcomposition or a mobility-controlling composition is performed inaccordance with a number of different preparation and injectionsequences. In accordance with one sequence, the foamed gel is preparedby premixing a gelation solution at the surface containing all of thefoamed gel components exclusive of the added gas. The added gas is thendispersed within the gelation solution under foaming conditions eitherbefore, after, or during injection of the gelation solution into thewellbore.

Addition of the gas to the gelation solution before injection of thegelation solution into the wellbore generates a preformed foamed gel atthe surface. The preformed foamed gel is subsequently injected into thewellbore and displaced into the formation. If the foamed gel is immatureupon displacement into the formation, it is aged to maturity therein. Ifthe foamed gel is mature upon displacement into the formation, nofurther aging of the foamed gel is required.

Addition of the gas to the gelation solution in situ after injection ofthe gelation solution into the wellbore generates the foamed gel insitu, rendering displacement of the foamed gel into the formationsubstantially simultaneous with generation thereof. Thus, for example,the gelation solution and gas can be sequentially injected into thewellbore and the foamed gel generated from the foamed gel components insitu upon or after entering the formation followed by aging of thefoamed gel, if required.

Addition of the gas to the gelation solution during injection of thegelation solution into the wellbore is achieved by coinjection of thegas and gelation solution into the wellbore. The foamed gel is eithergenerated in the injection means before the foamed gel enters theformation, or, alternatively, in situ after the foamed gel componentsenter the formation.

In accordance with an alternate sequence, a foamed gel having theabove-described composition is prepared by premixing a polymer-enhancedfoam at the surface containing the foamed gel components exclusive ofthe crosslinking agent. The crosslinking agent is then metered into thepolymer-enhanced foam during injection of the foam into the wellbore.The resulting immature foamed gel is then aged to maturity in thewellbore or the formation.

The above-described foamed gel has utility in a process for permeabilityreduction or in a process for mobility control, depending on therequirements of the formation in which the foamed gel is applied. Thedesign and practice of a process for permeability reduction or mobilitycontrol is within the purview of the skilled artisan applying theteaching set forth herein. The permeability reduction process hasapplications to a number of treatments, including conformanceimprovement treatments, water or gas shut-off treatments and water orgas coning treatments. Shut-off and coning treatments have particularutility at production well bores. Conformance improvement treatmentsimprove vertical and areal conformance within a subterranean formationand correspondingly improve flow profiles and sweep efficiencies ofinjected fluids or produced hydrocarbon fluids in the formation, therebyenhancing contact within the formation between injected fluids andproduced hydrocarbon fluids to desirably mobilize the hydrocarbon fluidsand facilitate their displacement from the formation.

The mobility control process promotes improved sweep efficiency andincremental oil recovery of a gas or liquid drive fluid in ahydrocarbon-bearing formation. The mobility control process is practicedby injecting the mobility-controlling composition into the formation ina flowing state ahead of, interspersed with, or behind the drive fluid.The mobility-controlling composition can also be injected into theformation via repeated sequential injection in accordance with anynumber of sequences incorporating the drive fluid.

Although the present invention is described as a permeability reductionprocess "or" as a mobility control process, it is understood that theterm "or" encompasses practicing the two processes simultaneously withinthe same formation using essentially the same foamed gel composition, aswell as practicing either of the two processes by itself as a separateand distinct process within the formation exclusive of the otherprocess. The following examples demonstrate the practice and utility ofthe present invention, but are not to be construed as limiting the scopethereof.

EXAMPLES

A number of foamed gel samples are individually prepared in samplebottles in accordance with the present invention. The sample bottles are0.2 liter, cylindrical, wide-mouth, glass bottles having an insidediameter of 4.5 cm. The samples are initiated by formulating gelationsolutions comprising PHPA, chromic trichloride, polyvinyl alcohol (PVA)and tap water. The gelation solutions are sparged with a gas to form thefoamed gel samples. One or more of the foamed gel parameters are variedbetween each sample for purposes of comparison. In all cases, however,the PHPA is the same composition having a molecular weight of about11,000,000 and 2 mole % hydrolysis. The pH of the foamed gel is alsomaintained slightly basic in all cases at about 8, adding a chemical pHadjuster if required to maintain the pH level. Unless otherwise notedthe gas is nitrogen.

The foamed gel samples are observed in the sample bottles throughout anaging time and characterized in accordance with the code set forthbelow. The results of the observations are reported in Examples 1-6.Unless otherwise noted concentration (conc) is reported in parts permillion (ppm), time is reported in hours (hr) and aging is performed atroom temperature.

Code for Characteristics of Foamed Gels

A No detectable gel formation. Same viscosity as initial uncrosslinkedpolymer solution.

B High flowing foamed gel. Slightly more viscous than initialuncrosslinked polymer solution.

C Flowing foamed gel. Most of foamed gel flows to bottom of sample jarupon inversion.

D Moderately flowing foamed gel. Small portion of foamed gel does notreadily flow to bottom of sample jar upon inversion.

E Barely flowing foamed gel. Foamed gel slowly flows to bottom of samplejar upon inversion or a significant portion does not flow to bottom uponinversion.

F Highly deformable nonflowing gel. Foamed gel flows to just short ofbottom of sample jar upon inversion.

G Moderately deformable nonflowing foamed gel. Foamed gel flows halfwayto bottom of sample jar upon inversion.

H Slightly deformable nonflowing foamed gel. Foamed gel surface onlyslightly deforms upon inversion of sample jar.

I Rigid foamed gel. No surface deformation of foamed gel upon inversionof sample jar.

* "+" or "-" denote shades of foamed gel strength at a given code level.

Example 1

    ______________________________________                                        Run No.  1        2        3      4      5                                    ______________________________________                                        polymer conc:                                                                          5000     5000     5000   2500   2500                                 cation conc:                                                                           164      164      164    41     41                                   PVA MW:  49,000   96,000   125,000                                                                              96,000 125,000                              PVA conc:                                                                              5000     5000     6700   5000   5000                                 ______________________________________                                        Foamed Gel Code                                                               Time:                                                                         ______________________________________                                         0        A        A       A      A     A                                     24        H        H       H      D     D                                     48        H        H       H      D     D                                     72        H        H       H      D     D                                     150       H        H       H      D     D                                     ______________________________________                                        Run No.          6       7                                                    ______________________________________                                        polymer conc:    5000    2500                                                 cation conc:     164     41                                                   PVA MW:          125,000 125,000                                              PVA conc:        5000    4000                                                 ______________________________________                                        Foamed Gel Code                                                               Time:                                                                         ______________________________________                                         0               A           A                                                24               H           D                                                48               H           D                                                72               H           D                                                150              H           D+                                               ______________________________________                                    

The results of Example 1 show that foamed gel strength increases withsing polymer concentration and that foamed gels can be formed with awide of PVA molecular weights.

Example 2

Example 2 is performed in a similar manner to Example 1 except thatdehyde (37% active) is added to the gelation solutions of Example 2 as aat a final active concentration of 1000 ppm.

    ______________________________________                                        Run No.  1       2       3     4     5     6                                  ______________________________________                                        polymer conc:                                                                          5000    5000    5000  2500  2500  2500                               cation conc:                                                                           164     164     164   41    41    41                                 PVA MW:  49,000  96,000  125,000                                                                             96,000                                                                              125,000                                                                             125,000                            PVA conc:                                                                              5000    5000    6700  5000  10,000                                                                              5000                               ______________________________________                                        Foamed Gel Code                                                               Time:                                                                         ______________________________________                                         0       A       A       A     A     A     A                                  24       H       H       H     H     H     H                                  48       H       H       H     H     H     H                                  72       H       H       H     H     H     H                                  150      H       H       H     H     H     H                                  ______________________________________                                    

The results of Example 2 show that the use of formaldehyde in the foamedgel as a biocide does not interfere with foamed gel formation and thatfoamed gels can be formed with a wide range of PVA concentrations.

Example 3

Example 3 is performed in a similar manner to Example 1 except thatformaldehyde (37% active) is added to the gelation solutions of Example3 as a biocide at a final active concentration of 1000 ppm. Example 3 isalso performed at temperatures exceeding room temperature.

    ______________________________________                                        Run No.     1       2         3     4                                         ______________________________________                                        temp (°C.)                                                                         51      59        64    67                                        polymer conc:                                                                             5000    5000      2500  2500                                      cation conc:                                                                              164     82        41    82                                        PVA MW:     125,000 125,000   125,000                                                                             125,000                                   PVA conc:   5000    2900      4000  2900                                      ______________________________________                                        Foamed Gel Code                                                               Time:                                                                         ______________________________________                                         0          A        A        A      A                                        24          H        H        D      D                                        48          H        H        E      E                                        72          H        H        E      E                                        150         H        H        E      E                                        ______________________________________                                    

The results of Example 3 show that foamed gels can be formed across awide range of temperatures.

Example 4

Example 4 is performed in a similar manner to Example 1 except that thegelation solutions of Example 4 are sparged with ambient air rather thannitrogen.

    ______________________________________                                        Run No.     1       2         3     4                                         ______________________________________                                        polymer conc:                                                                             2500    5000      2500  2500                                      cation conc:                                                                              41      82        82    82                                        PVA MW:     125,000 125,000   125,000                                                                             125,000                                   PVA conc:   10,000  2900      4000  2900                                      ______________________________________                                        Foamed Gel Code                                                               Time:                                                                         ______________________________________                                         0          A        A        A      A                                         1          D        G        D      D                                        24          G        H        D      D                                        96          H        H        E      E                                        150         H        H        E      E                                        ______________________________________                                    

Example 5

Example 5 is performed in a similar manner to Example 1 except thatformaldehyde (37% active) is added to the gelation solutions of Example5 as a biocide at a final active concentration of 1000 ppm. The gelationsolutions of Example 5 are also sparged with ambient air rather thannitrogen.

    ______________________________________                                        Run No.     1       2         3     4                                         ______________________________________                                        polymer conc:                                                                             5000    2500      2500  2500                                      cation conc:                                                                              82      41        82    82                                        PVA MW:     125,000 125,000   125,000                                                                             125,000                                   PVA conc:   10,000  2900      4000  2900                                      ______________________________________                                        Foamed Gel Code                                                               Time:                                                                         ______________________________________                                         0          A        A        A      A                                         1          D        G        D      D                                        24          D        H        D      D                                        96          E        H        E      E                                        150         E        H        E      E                                        ______________________________________                                    

Example 6

As a control experiment, Example 6 is performed in a similar manner toExample 1 except that PVA is omitted from the gelation solutions ofExample 6.

    ______________________________________                                        Run No.   1       2       3    4      5    6                                  ______________________________________                                        polymer conc:                                                                           10,000  5000    2500 10,000 5000 2500                               cation conc:                                                                            164     82      41   164    82   41                                 ______________________________________                                    

The results of Example 6 show that no foaming of the produced gelsoccurs in the absence of PVA, indicating that PVA is an essentialfunctional component in the foamed gels of the present invention.

Example 7

As a control experiment, PVA solutions are prepared in the samplebottles, omitting all other gelation solution components. The PVAsolutions are sparged with nitrogen and the resulting foam samples areaged and characterized in accordance with the code set forth below. Theresults of the observations are reported below and unless otherwisenoted aging is performed at room temperature and time is reported inhours (hr).

Code for Characteristics of Foams

A--no foam formed

B--foam bubbles only appear near surface of the solution

C--foam bubbles only appear throughout top half of the solution

D--foam bubbles appear throughout the entire solution, but nosignificant height increase in level of solution

E--entire solution foamed, small increase in level of solution (≦2.5 cm)

F--entire solution foamed, foam fills half of sample jar

G--entire solution foamed, foam fills entire sample jar

* "+" or "-" denote shades of foaming at a given code level.

    ______________________________________                                        Run No.  1       2       3     4     5     6                                  ______________________________________                                        PVA MW:  96,000  96,000  96,000                                                                              125,000                                                                             125,000                                                                             125,000                            PVA conc:                                                                              20,000  13,300  10,000                                                                              20,000                                                                              13,300                                                                              10,000                             ______________________________________                                        Foam Code                                                                     Time:                                                                         ______________________________________                                        0         G       G       G     G     G     G                                 10 min.   F-      F-      F-    F     F     F                                 20 min.   B       B       B     F     F-    E+                                30 min.   A       B-      B-    F     E+    E                                 40 min.   A       A       A     F-    E     E                                 50 min.   A       A       A     F-    E-    E-                                1                               F-    E-    E-                                1.5                             E+    B+    B+                                2                               E     B     B                                 2.5                             E     B     B                                 3                               E-    B     B                                 3.5                             B+    B-    B-                                4                               B+    B-    B-                                4.5                             B     A     A                                 5                               A     A     A                                 ______________________________________                                    

The results of Example 7 show that no usable foamed gel is produced inthe absence of PHPA and chromic trichloride.

In summary, it is apparent from Examples 1-5 that stable foamed gels areproduced in accordance with the present invention. Example 6, ascompared to Examples 1-5, demonstrates that PVA possessessurfactant-like properties that are required to produce the foamed gelsof the present invention. Example 7 demonstrates that a suitablecrosslinkable polymer and crosslinking agent are likewise required toproduce the foamed gels of the present invention.

While the foregoing preferred embodiments of the invention have beendescribed and shown, it is understood that alternatives andmodifications, such as those suggested and others, may be made theretoand fall within the scope of the present invention.

I claim:
 1. A process for reducing the permeability of a treatmentregion in a subterranean hydrocarbon-bearing formationcomprising:placing a foamed gel in a treatment region of a subterraneanhydrocarbon-bearing formation to substantially reduce the permeabilityof said treatment region, wherein said foamed gel is formed from anadded gas and a gelation solution including an acrylamide-containingcrosslinkable carboxylate-containing polymer, a crosslinking agentcontaining a reactive transition metal cation, a foam promotingcomposition consisting of a single chemical species, and an aqueoussolvent, wherein said single chemical species of said foam promotingcomposition is a polyvinyl alcohol and said polyvinyl alcohol has aconcentration in said gelation solution from about 20 ppm to about100,000 ppm and has an average molecular weight from about 1,000 toabout 500,000.
 2. The process of claim 1 wherein said foamed gel isprepared by contacting said gelation solution with said added gas, andsubstantially crosslinking said polymer with said crosslinking agent. 3.The process of claim 2 wherein said gelation solution and added gas arecontacted at the surface and thereafter injected into a wellbore influid communication with said formation.
 4. The process of claim 2wherein said gelation solution and added gas are contacted at thesurface by coinjection into a wellbore in fluid communication with saidformation.
 5. The process of claim 2 wherein said gelation solution andadded gas are contacted in said formation.
 6. The process of claim 1wherein said acrylamide-containing polymer is selected from a groupconsisting of polyacrylamide, partially hydrolyzed polyacrylamide, andcopolymers, terpolymers and tetrapolymers of acrylamide.
 7. The processof claim 1 wherein said reactive transition metal cation is trivalentchromium.
 8. The process of claim 1 wherein said crosslinking agent isan organic salt of trivalent chromium.
 9. The process of claim 1 whereinsaid crosslinking agent is a complex including trivalent chromium andone or more anionic carboxylate ligands.
 10. The process of claim 9wherein said one or more anionic carboxylate ligands is acetate.
 11. Theprocess of claim 1 wherein said gelation solution further includes abiocide.
 12. The process of claim 1 wherein said gelation solutionfurther includes a delaying agent.
 13. A process for controlling themobility of a drive fluid in a subterranean hydrocarbon-bearingformation comprising:placing a foamed gel in a subterraneanhydrocarbon-bearing formation in association with a drive fluid tocontrol the mobility of said drive fluid in said formation, wherein saidfoamed gel is formed from an added gas and a gelation solution includingan acrylamide-containing crosslinkable carboxylate-containing polymer, acrosslinking agent containing a reactive transition metal cation, a foampromoting composition consisting of a single chemical species, and anaqueous solvent, wherein said single chemical species of said foampromoting composition is a polyvinyl alcohol and said polyvinyl alcoholhas a concentration in said gelation solution from about 20 ppm to about100,000 ppm and has an average molecular weight from about 1,000 toabout 500,000.
 14. The process of claim 13 wherein said foamed gel isprepared by contacting said gelation solution with said added gas, andsubstantially crosslinking said polymer with said crosslinking agent.15. The process of claim 14 wherein said gelation solution and added gasare contacted at the surface and thereafter injected into a wellbore influid communication with said formation.
 16. The process of claim 14wherein said gelation solution and added gas are contacted at thesurface by coinjection into a wellbore in fluid communication with saidformation.
 17. The process of claim 14 wherein said gelation solutionand added gas are contacted in said formation.
 18. The process of claim13 wherein said acrylamide-containing polymer is selected from a groupconsisting of polyacrylamide, partially hydrolyzed polyacrylamide, andcopolymers, terpolymers and tetrapolymers of acrylamide.
 19. The processof claim 13 wherein said reactive transition metal cation is trivalentchromium.
 20. The process of claim 13 wherein said crosslinking agent isan organic salt of trivalent chromium.
 21. The process of claim 13wherein said crosslinking agent is a complex including trivalentchromium and one or more anionic carboxylate ligands.
 22. The process ofclaim 21 wherein said one or more anionic carboxylate ligands isacetate.
 23. The process of claim 13 wherein said gelation solutionfurther includes a biocide.
 24. The process of claim 13 wherein saidgelation solution further includes a delaying agent.